Interaction of hydrogen peroxide and chlorine dioxide stages in ecf bleaching, TAPPI JOURNAL, June 1998, Vol. 81(6)

David J. Senior, Janice Hamilton, Arthur J. Ragauskas, James Sealey, Peter Froass

Changing the order of the hydrogen peroxide (P) and chlorine dioxide (D) stages in an ECF (elemental chlorine free) bleach sequence was shown to affect the final brightness of kraft pulps. Application of a D(EOP)DPP bleach sequence to a southern soft wood pulp produced consistently higher interstage and final brightnesses than a D(EOP)PDP sequence. These results were confirmed by bleaching a northern softwood and a eucalypt pulp using DEDP and DEPD sequences. In all cases, higher brightnesses were achieved when the second D stage (D2) preceded the P stage. The effect was especially evident with the hardwood pulp, where brightness of the DEDP pulp was three points higher then the DEPD pulp. NMR analysis of extraacted lignin showed that the D2 stage reduced the phenolic lignin content of softwood pulp by 60% while the P stage had no effect. For the hardwood pulp, the D2 stage reduced the phenolic lignin content by 90-95%. The greater bleaching efficiency of a DEPD sequence (compared with DEPD) is consistenc with known reactivities of hydrogen peroxide and chlorine dioxide toward specific lignin structures. The D2 stage lowers the phenolic lignin content of pulp while generating hydrogen peroxide-sensitive, nonphenolic lignin species. A followup P stage reacts effectively with this lignin.